This invention relates generally to imaging systems capable of operation in multiple modalities, and more particularly to methods and systems for maintaining an alignment of the object being scanned.
Multi-modality imaging systems are capable of scanning using different modalities, such as, for example, Positron Emission Tomography (PET), Single Positron emission tomography (SPECT), Ultrasound, Magnetic Resonance Imaging (MRI), and Computed Tomography (CT), Static X-Ray imaging, and Dynamic (Fluoroscopy) X-Ray imaging. In a multi-modal system (also referred to as a multi-modality system), a portion of the same hardware is utilized to perform different scans (e.g., an image produced by SPECT is processed and displayed respectively, by the same computer and display, as an image produced by CT). However, the data acquisition systems (also referred to as an “imaging assembly”) are different. For example, on a CT/SPECT system, a radiation source and a radiation detector are used in combination to acquire CT data, while a radiopharmaceutical is typically employed in combination with a SPECT camera to acquire SPECT data.
In multi-modality systems, such as, for example, an integrated SPECT/CT system there is an inherent registration of the SPECT and CT images the system acquires. Since the patient lies still on the same table during the SPECT and CT portions of the acquisition, the patient will be in a consistent position and orientation during the two acquisitions, greatly simplifying the process of correlating and fusing the CT and SPECT images. This allows the CT image to be used to provide attenuation correction information for the reconstruction of the SPECT image, and allows an image reader to easily correlate the anatomic information presented in the CT image and the functional information presented in the SPECT image.
This inherent registration assumes an alignment of the SPECT and CT detector coordinate systems, or at least a known spatial transformation between the two coordinate systems. A misalignment of the coordinate systems may directly result in a misregistration of the images. Misregistration leads not only to wrong localization but also to wrong attenuation correction of the functional image.
Proper SPECT and CT image registration also requires an alignment of the axial (z-) axis of the SPECT and CT coordinate systems not only with each other, but also with the travel axis of the table that transports the patient between and during the SPECT and CT acquisitions. A co-axial SPECT/CT or other multi-modality system, especially for whole body scans, requires a relatively long axial travel distance to permit both imaging modalities the ability to image the region of interest. However, a patient table and table support may not be able to accommodate the alignment requirements while supporting a patient cantilevered out from the table support during an examination due to the extreme length of travel the patient table must travel to reach both imaging assemblies. For example, a co-axial imaging assembly arrangement requires a relatively long rail system, and the length of the bed may induce bending thereof, such that the patient position may change between the two imaging stations, even if the patient remains absolutely stationary.